Equipment for generating a rotating laser beam, generally disposed in a horizontal plane, or a plane tilted at a known number of degrees from the horizontal, is now widely used in the construction and earth working industries. As described in Studebaker U.S. Pat. No. 3,588,249 such transmitters generally involve a tripod support for a laser beam generator which produces a vertically directed beam. The beam is caused to impinge upon a generally horizontal surface of a rotating pentaprism and the beam emitted from the pentaprism is exactly at a ninety degree angle with respect to the incident beam and sweeps through a generally horizontal plane, which will be exactly horizontal if the incident generated beam is exactly vertical. Three thumb screws and two mutually perpendicular levels are generally employed on the transmitter to effect vertical adjustment of the emitter laser beam to cause it to lie exactly in a vertical plane, thus resulting in the rotating beam being exactly horizontal.
In many applications, for example, when utilizing the rotating laser beam to control ditching equipment, it is preferrable that the plane defined by the rotating laser beam be tilted from the horizontal by a known number of degrees corresponding to the desired pitch or fall of the drainage trench being dug. This has been conveniently accomplished in the past by employing sine bar type adjusting mechanisms for the bubble type levels provided on the transmitter. The plane defined by rotating beam may then be either initially adjusted to lie in an exactly horizontal plane or in any plane tilted by a known number of degrees with respect to the horizontal.
It is obviously desirable that once the plane of the rotating laser beam has been determined, the beam should continue to maintain the plane throughout the working period of the equipment that it is controlling or the surveying operation for which it is being used as reference plane. Unfortunately, in the course of a normal working day, temperature changes, changes in wind velocity and ground vibration due to the passage of heavy equipment near the transmitter, all can have the effect of disturbing the initial adjustment of the plane of the rotating laser beam. It has therefore been necessary for the operators to make periodic checks of the planar position of the rotating laser beam in order to assure that it has not drifted from the desired setting. Since such checks must necessarily be periodic, this opens the opportunity for the laser beam reference plane being in error for a substantial period of time without knowledge of the equipment operator or the surveying crew. It is therefore apparent that any mechanism which would continuously monitor the position of the rotating laser beam to determine if it did in fact correspond with the desired position and then, when a departure was detected, automatically effect a correction of the plane defined by the rotating laser beam back to the initial desired position, would be a highly desirable and advantagous improvement in this type of equipment.
Accordingly, it is an object of this invention to provide a method and apparatus for continuously monitoring the position of the plane defined by the rotating laser beam to generate a signal whenever such plane departs from a desired position.
A further object of this invention is to provide a method and apparatus for automatically effecting a correction of the plane defined by a rotating laser beam to restore such plane to a desired position anytime that the beam tends to depart from the desired position.